The field of the invention relates generally to semiconductor devices and more specifically, to lithography.
In an effort to fit more device functionality into smaller areas and to increase the speed of integrated circuits, the dimensions of features (e.g. widths of interconnect lines) in integrated circuits are shrinking. One obstacle to overcome is the reliability of forming a desired pattern with small features in a photoresist layer by shining radiation (e.g. light) through a reticle, a process termed optical lithography, at a reasonable cost. Since optical lithography is diffraction limited, the smallest feature that can be printed in the photoresist layer is constrained by the phenomena that as light travels through an opening in the reticle, the light spreads out (diffracts). If the diffracted light is not captured by the imaging optics, pattern information is lost and the small features cannot be constructed in the photoresist pattern. Since imaging optics inherently contain flaws, they cannot capture all of the pattern information. Thus, it is desirable to minimize diffraction.
One approach is optical phase shifting lithography, which uses a reticle with a patterned transparent material that has a predetermined thickness so light transmitted through the transparent material is 180 degrees out of phase with neighboring areas, which do not include the transparent material. The resultant interference effects improve the contrast, resolution and other process parameters of the pattern.
Many different approaches to optical phase shifting lithography are used to shift the phase of the light in predetermined portions of a reticle relative to other portions of the reticle in an effort to decrease diffraction and print small features. One type of mask is an alternating phase shifting mask (APSM), which has only 0 degree and 180 degree phase shifting regions. A major disadvantage of the APSM is that at the boundaries between the 0 degree and 180 degree phase shifting regions phase conflicts arise, which can lead to undesirable printing artifacts such as a line that is not part of the desired pattern.
Another optical lithography approach, a complementary phase shifting mask (CPSM) seeks to prevent the undesirable phase-conflict effects arising from the APSM by adding a second (non-phase shifting) mask, which is complementary to the first (phase shifting) mask. Although the phase conflict problem is solved, using the CPSM increases cycle time, cost and manufacturing complexity because two masks are used to form the desired pattern.
To overcome the disadvantages of APSM and CPSM, a rim phase shifting mask (RPSM) is used. A RPSM has rims located along edges of an opaque patterned region, which is usually chrome and is formed over a quartz substrate. The opaque patterned region blocks light so that a photoresist layer on a semiconductor wafer is not developed in areas underneath the opaque patterned region when using the RPSM in a lithographic process. Typically, the rims are trenches in the quartz substrate that shift the light 180 degrees relative to the quartz substrate to enhance the image contrast of the opaque patterned region, thereby improving resolution and process margin of the corresponding feature in photoresist on a semiconductor wafer.
One method to form the RPSM includes coating a photoresist layer over an unpatterned chrome layer that has been deposited over a planar quartz substrate. The first photoresist layer is patterned and used as a mask to first etch the chrome and then etch into the quartz to form the rims. It is important that the etching of the quartz is well controlled because the rims are extremely small (approximately 10-20% the size of the adjacent patterned chrome). After forming the rims, the first photoresist layer is removed and a second photoresist layer is coated over the chrome layer and patterned. The chrome layer is etched using the second photoresist pattern as a mask. The alignment of the second photoresist layer to the rims in the quartz substrate should allow remaining portions of the chrome layer to lie between the rims after the chrome has been etched back from the quartz rims. The above method also is problematic because two lithographic processes (i.e. photoresist patterning steps) are performed, which increases manufacturing time and cost. Therefore, a need exists for a controllable method to form a RPSM that self-aligns the rims to the patterned chrome and minimizes manufacturing time and cost.